Measure the voltage at the drive and at the battery as you disconnect the charger. It's possible you have one or more weak cells in the battery causing the voltage to sag rapidly as soon as the charger is disconnected. The battery meter likely won't know how to deal with this, as it expects a battery in reasonably good condition. I don't think battery packs in the era of the Portable had much if any build in intelligence to assess the health of the cells the way modern ones do.